There are a number of technological applications which require cooling to low temperatures and in particular cryogenic temperatures which may be thought of as those below 100 Kelvin. Liquid helium-4 is often used as a cryogenic coolant due to its boiling point at atmospheric pressure of around 4 Kelvin. Superconducting magnets and other experimental devices are traditionally cooled to around 4 Kelvin using liquid cryogens, these including nitrogen and helium. The relatively large enthalpy content of these cryogens in either liquid or gaseous form ensures a rapid cooling from room temperature down to that of the cryogen in question. Despite the widespread use and success of liquid cryogens, the apparatus necessary to handle such low temperature liquids is often rather bulky, complicated and expensive. Furthermore, the relative scarcity of helium increasingly makes the use of this cryogen unfavourable.
Thus there has been a general trend towards the reduction of the volumes of liquid cryogens used, their cooling power being replaced by mechanical cryo-coolers (“mechanical refrigerators” herein), these include pulse-tube coolers, Gifford McMahon and Stirling coolers. Recent developments in double-staged mechanical refrigerators have enabled a more cost-effective and convenient cooling procedure. However, one particular disadvantage of such mechanical refrigerators is that the relatively small cooling power of the second stage (the lower temperature of the two stages) means that it takes significantly longer to cool an apparatus down using mechanical refrigerators in comparison with liquid cryogens. The greater the thermal mass of the target being cooled, the greater the disadvantage of using mechanical refrigerators because of their low cooling power at low temperatures.
There is a strong desire to improve the cooling power of mechanical refrigerators which would enable practical use of such apparatus in applications for which at present they are not considered available. In some applications, notably high field superconducting magnets, it is expected that the pursuit of ever higher magnetic fields will mean an increase in the thermal mass of the magnets in question and therefore there is a need to improve the cooling performance of mechanical refrigerators if they are to remain useful in cooling superconducting magnets from room temperature to their operating temperature.